I. Field of the Invention
This invention relates generally to the measurement of radioactivity of a sample in a container and, more particularly to a new and improved sample container for either wet or dry samples.
II. Background Description
In the biological and medical sciences, certain radioisotopes are frequently used as tracers in tests and experiments in order to detect minute quantities of certain biochemicals present in test samples. For example, the radioisotope .sup.32 P, is commonly used by researchers in these fields to label genetic material (DNA/RNA) and proteins. Frequently, it is required to know the precise amount of a radioisotope contained in various test samples. Quantitative measurements of the amount of radioactive material present in a test sample usually are expressed as an activity in disintegrations per minute. These measurements provide valuable information both in preparing the radio-labelled chemicals and in measuring an amount of radio-labelled material recovered from a system under investigation. Measurements of the activity of a sample also are needed to limit exposure to personnel handling the radioisotopes.
At the present time, most measurements of activity are obtained from scintillation counting or from Geiger counting. Scintillation counting uses photomultiplier tubes to detect photons produced in a scintillation medium in response to absorption by the medium of beta and gamma radiation. Many of the photons emitted from the scintillation medium are incident upon a photocathode of a multiplier phototube. These photons are converted to photoelectrons and are multiplied in number at a succession of phototube electrodes, called dynodes, the output of which is a measurable electrical pulse related to the incident radiation.
Liquid scintillation counters operate on the same basic principle as scintillation counters, except that the scintillation medium is a liquid into which is dissolved, suspended or otherwise intermixed the radioactive sample being tested. Radioactive emissions of a sample are measured by collecting photons emitted from the scintillation medium and generating photoelectrons responsive thereto to produce electrical pulses related to the incident beta and gamma radiation.
Scintillation and liquid scintillation counting require special sample preparation and the use of special sample containing vials in order to provide a quantitative measure of the amount of radioactive material present in a particular sample. Accordingly, an extra material handling step, involving a transfer of radioactive material into one of the special vials, is required when using these techniques. This transfer step is undesirable, for it is accompanied by an element of error in the measurement of material transferred to the vials. When this measurement error is added to the error inherent to the particular experiment or test technique being utilized, further uncertainty as to the accuracy of the quantitative data obtained from the sample results. Furthermore, the preparation of even a small amount of material for scintillation counting results in the loss of that material for further experimentation. In many cases, where only a very limited quantity of material is available this loss may be unacceptable.
Geiger counters are generally used when counting small numbers of samples. These counters provide a simpler but much less reliable means for measuring an approximate activity of a radiation emitting sample. Geiger counters use gas filled tubes the contents of which are ionized by incident radiation to produce an electronic signal which registers on a meter or in an audio circuit. The magnitude of the electronic signal is proportional to the amount of radiation impinging upon the gas filled tubes. Commercial Geiger counters are generally hand held devices whose quantitative accuracy is limited by uncertainties in the geometrical positioning of the sample relative to the detector and the absence of careful calibration techniques. However, the instruments are very useful in determining the presence and/or location of radioactivity and in determining an approximate activity of the sample for safe handling considerations. Geiger counters are also helpful in assessing the progress of certain chemical reactions or experiments.
At the current time, the vast majority of low energy radioisotope samples are counted using the technique of scintillation counting. The lowest energy samples such as tritium (.sup.3 H), carbon-14 (.sup.14 C), sulfur-35 (.sup.35 S), and phosphorus-32 (.sup.32 P) are counted using liquid scintillation counting (LSC). Liquid samples to be counted are placed in standard LSC vials, mixed with scintillation chemicals which fluoresce when excited by the low energy radiation, and counted in an instrument which detects the light flashes produced inside the vials. In the case of the highest energy of these isotopes (.sup.32 P), Cerenkov light is also produced by the interaction of the radiation with water or glass without the addition of a special scintillation chemical, and this light can also be counted by the LSC instrumentation.
For higher energy radiation, such as gamma emitters iodine-125 (.sup.125 I) and technetium-99m (.sup.99m TC), samples can be counted with a solid scintillation crystal. The gamma radiation is much more penetrating than the beta radiation of the isotopes cited above and can penetrate the walls of a container and enter a crystal detector which produces light in response to the emitted radiation. Again, the light flashes are counted by the instrumentation.
These techniques require samples in large standard vials (from approximately 10-25 cc volume). The vials are generally made from plastic or glass of sufficient thickness to absorb essentially all of the low energy beta emissions from carbon-14 and sulfur-35. For these low energy beta emitting isotopes, the samples must be mixed with a scintillation chemical to produce the desired light flashes. This chemical also destroys most biological activity, rendering the counted samples unsuitable for further biological experimentation. There are also problems of compatibility between sample and scintillant including phase separation and coloration which must be eliminated or measured to obtain accurate results. In addition, the samples once counted must be disposed of, and there are significant volumes of liquid radioactive waste generated.
Another type of sample preparation in the prior art is the planchette used with a planchette counter, proportional and Geiger counters. In these devices, the liquid sample is spread and dried on a metal or paper surface. After drying, it can then be introduced into the sensitive gas volume of a proportional or Geiger counter which counts the radioactivity.
A currently available detector is a compact, bench top radiation detection apparatus capable of measuring the radiation in samples placed in any of a plurality of different sized and shaped containers through the provision of removable sample holders configured to receive different sample containers. Thus, the apparatus can measure the radiation from sample containers. Thus, the apparatus can measure the radiation from samples in the form of a liquid in a vial when the energy of the emissions are sufficient to penetrate the wall of the vial, as well as from samples which have been deposited and dried on the surfaces of specially shaped disposable sample containers which can be inserted into an associated sample holder and positioned at a fixed distance from a radiation detector with the sample in direct communication with the detector. This latter arrangement enables an efficient and accurate measurement to be made due to the absence of a container wall between the sample and the detector. It also allows the apparatus to be used to detect and measure low energy emissions. The vials however, have the limitations mentioned above.
The foregoing illustrates limitations known to exist in present devices. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.